In a flow control valve used in an EGR system, a valve seat is formed at an interface of two cylinder members press-fitted into a gas passage (for example, see Patent Literature 1). FIG. 26 shows a sectional view of a flow control valve of the same type. As shown in FIG. 26, a flow control valve 100 includes a housing 101, a first cylinder member 102, a second cylinder member 103, and a valve body 111. These members are all formed of stainless steel. A gas passage 107 is formed inside the housing 101. The gas passage 107 includes a first cylinder member accommodating portion 109, a second cylinder member accommodating portion 110, and a radial step portion 108.
The first cylinder member 102 is accommodated in the first cylinder member accommodating portion 109. The second cylinder member 103 is accommodated in the second cylinder member accommodating portion 110. The first cylinder member 102 and the second cylinder member 103 are continuous in the axial direction in a state where inner peripheries thereof are displaced with each other in the vicinity of the radial step portion 108. A valve seat 104 is formed around the whole circumference at the interface between the first cylinder member 102 and the second cylinder member 103. The valve seat 104 includes a first area 105 and a second area 106. The first area 105 is arranged at about 180° portion of the inner periphery of the first cylinder member 102. The second area 106 is arranged at about 180° portion of the inner periphery of the second cylinder member 103. The valve body 111 detaches from and attaches to the first area 105 and the second area 106 (i.e., the valve seat 104) to open and close the gas passage 107. The flow rate of exhaust gas is adjusted by the valve body 111 turning in the gas passage 107.